The present invention relates generally to communication systems. More particularly, the present invention relates to the stabilization of a signal envelope for communication systems.
Typical communication systems transmit information from one location or source to a second location or destination. The information travels from the source to the destination through a channel; this channel is typically a noisy channel. Thus, the channel introduces various forms of noise. The term “noise” is used herein to define various forms of signal corruption, such as interference, faxing, attenuation, environmental impact, and electronic noise, that alter the characteristics of a signal as it travels through a channel. Accordingly, the signal that is transmitted through the channel and received at a receiver may be a combination of the transmitted signal and the effects of noise introduced by the channel as a result of travelling through the channel.
In a cellular communications system, one type of noise is called “interference”. More specifically, there may be at least two forms of interference in communication systems: co-channel interference (CCI) and inter-symbol interference (ISI). CCI arises in communication systems due in part to the fact that there are several transmitters in communication with the same receiving unit. The signal from one transmitter may interfere with the signal from another transmitter. Each transmitter may be an omni-directional transmitter. However, a signal being transmitted from one transmitter may take several paths as the signal travels from the transmitter to the receiver. This leads to ISI, which is a form of self-interference. In a cellular communication system, there are several mobile stations in communication with the same base station which often leads to CCI.
As indicated above, in a communication system, information is transmitted through the channel from the source to the destination. The information may be carried by a carrier signal that is modulated to contain or carry the information. Various forms of modulation may be used for transmission of the information through the channel. Modulation is the process of varying the characteristic of a carrier according to an established standard or scheme; the carrier is prepared or “modulated” by the information to produce a “modulated” carrier signal that is transmitted by the source to the destination through the channel. For example, in a cellular communication system, modulation is the process of varying the characteristics of the electrical carrier as information is being transmitted. The most common types of modulation are Frequency Modulation (FM), Amplitude Modulation (AM), and Phase Modulation (PM).
One modulation technique currently used in the industry is called Orthogonal Frequency Division Multiplexing (OFDM). OFDM is one of the techniques for multi-carrier modulation. Multi-carrier modulation is a technique for modulating multiple carriers with different information, all of which may be transmitted simultaneously or parallel in time. OFDM has high spectral efficiency as well as tolerance to multipath fading. As indicated above, transmitters are omni-directional and transmit in all directions. Thus, a signal emerging from a transmitter, or the source, may travel multiple paths to reach the receiver, or the destination. Accordingly, multipath fading occurs on a carrier signal's intensity, which results in alteration of the information being carried.
The efficiency of a system utilizing OFDM stems from the simultaneous or parallel transmission of several subcarriers in time. While this lowers the bit-rate on each of the subcarriers, it provides an “N”-fold increase in aggregate bit-rate, wherein “N” is the number of subcarriers. Additionally, because the low bit-rate signals hardly suffer any ISI and the subcarriers are orthogonal, it is possible to demodulate the subcarriers independent of each other. A conventional OFDM system comprises a set of sub-symbols X[k] transmitted in time using an Inverse Fast Fourier Transform (IFFT). The time-domain baseband signal can be represented as:
            x      ⁡              [        n        ]              =                  1                  N                    ⁢                        ∑                      k            =            0                                N            -            1                          ⁢                              X            ⁡                          [              k              ]                                ·                      exp            ⁡                          (                                                j2                  ⁢                                                                          ⁢                  π                  ⁢                                                                          ⁢                  kn                                N                            )                                            ,      n    =    0    ,            1      ⁢                          ⁢      …      ⁢                          ⁢      N        -    1  
Thus, the N-sample long transmitted OFDM symbol vector can be expressed as:xN=IFFT{XN}
where, xN and XN are the time and frequency domain symbol vectors, respectively.
In a typical OFDM system, binary symbols or bit streams are encoded in the form of complex valued numbers. The complex valued numbers are drawn from an M-ary alphabet. The complex valued numbers are then used to modulate a set of orthogonal sub-carriers to generate a time-domain signal using an Inverse Discrete Fourier Transform (IDFT). The resulting baseband signal, which is usually complex valued, is quadrature modulated on a Radio Frequency (RF) carrier and transmitted through an air interface channel. The transmitted signal is corrupted by channel noise and dispersion before being received. At the receiver end, by estimating the channel, equalizing for it and detecting the transmitted complex-valued numbers, the data is decoded.
There are several problems associated with systems that utilize OFDM modulation techniques. For example, the channel is subject to fading due to multipath and path loss. Additionally, the channel may suffer from ISI which poses a problem at the receiver when data has to be detected. Furthermore, manufacturers of devices that transmit and receive data are always faced with the challenge of increasing the amount of and the rate at which information can be transmitted over a finite bandwidth while overcoming signal loss due to channel noise.
Embodiments of the present invention are related to the implementation of Orthogonal Frequency Division Multiplexed (OFDM) systems. One of the persistent drawbacks of OFDM is the high peak-to-average power ratio (PAPR) encountered in OFDM systems. Coherent addition of the modulating sub-symbols can lead to an occasional peak in the signal that is several dB above average. A high PAPR usually implies that a very linear but inefficient power amplifier (PA) must be used for RF transmission. Furthermore, to make allowance for the high PAPR, we need to operate the power amplifier with several dbs of input power backoff; thus, limiting the average power of the output signal. Clipping is often the simplest solution proposed, but can lead to out-of-band distortion. Clipping also causes signal loss and often yields unacceptable bit error rates (BER). Other techniques considered have been block-coding and constellation translation. A high PAPR typically implies that the power amplifiers at the transmitter end and the amplifiers at the receiver end need to be of very high linearity, and preferably of high efficiency too. It is very common to trade-off high linearity for efficiency in OFDM type of applications so that often highly linear but inefficient amplifiers end up being utilized. An embodiment of the present invention enables the use of efficient amplifiers potentially by stabilizing the envelope of the RF carrier in an OFDM system. Alternatively, it enables the use of the traditional amplifiers with greater output power. Partial response signaling is employed to compress the signal in time. Then the power envelope is shaped or more specifically, squared.
It is in light of this background information related to high peak-to-average power ratio (PAPR) encountered in OFDM systems described above that the present invention has evolved.